Glycols such as mono-ethylene glycol (MEG) and mono-propylene glycol (MPG) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focussed on producing chemicals, including glycols, from non-petrochemical renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols can be seen as an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to glycols revolve around a two-step process of hydrogenolysis and hydrogenation, as described in Angew, Chem. Int. Ed. 2008, 47, 8510-8513.
Such two-step reaction requires at least two catalytic components. Patent application WO2015028398 describes a continuous process for the conversion of a saccharide-containing feedstock into glycols, in which substantially full conversion of the starting material and/or intermediates is achieved and in which the formation of by-products is reduced. In this process the saccharide-containing feedstock is contacted in a reactor vessel with a catalyst composition comprising at least two active catalytic components comprising, as a first active catalyst component with hydrogenation capabilities, one or more materials selected from+transition metals from groups 8, 9 or 10 or compounds thereof, and, as a second active catalyst component with retro-aldol catalytic capabilities, one or more materials selected from tungsten, molybdenum and compounds and complexes thereof. Retro-aldol catalytic capabilities referred to herein means the ability of the second active catalyst component to break carbon-carbon bonds of sugars such as glucose to form retro-aldol fragments comprising molecules with carbonyl and hydroxyl groups. Glucose, which is an aldol product, for example, when broken into simple retro-aldol fragments yields glycolaldehyde.
It is well known in the art of chemicals manufacturing that catalysts may be described as homogeneous or heterogeneous, the former being those catalysts which exist and operate in the same phase as the reactants, while the latter are those that do not.
Typically, heterogeneous catalysts may be categorised into two broad groups. One group comprise the supported catalytic compositions where the catalytically active component is attached to a solid support, such as silica, alumina, zirconia, activated carbon or zeolites. Typically these are either mixed with the reactants of the process they catalyse, or they may be fixed or restrained within a reaction vessel and the reactants passed through it, or over it. The other group comprise catalytic compositions where the catalytically active component is unsupported, i.e. it is not attached, to a solid support, an example of this group is the Raney-metal group of catalysts. An example of a Raney-metal catalyst is Raney-nickel, which is a fine-grained solid, composed mostly of nickel derived from a nickel-aluminium alloy. The advantage of heterogeneous catalysts is that they can be retained in the reactor vessel during the process of extracting the unreacted reactants and the products from the reactor vessel, giving the operator the capability of using the same batch of catalysts many times over. However, the disadvantage of heterogeneous catalysts is that over time their activity declines, for reasons such as the loss or leaching of the catalytically active component from its support, or because the access of the reactants to the catalytically active component is hindered due to the irreversible deposition of insoluble debris on the catalyst's support. As their activity declines, catalysts need to be replaced, and for heterogeneous catalysts this inevitably requires the process that they catalyse to be stopped, and the reactor vessel to be opened up, to replace the deactivated catalyst with a fresh batch. Such down-time is costly to the operators of the process, as during such time no products can be produced, and such labour-intensive operations have cost implications.
A further complication of using heterogeneous catalysts is that the process of preparing the catalyst, and in particular the process of immobilising catalytically active components onto a solid support in a way that gives maximum catalytic activity can be difficult and time consuming.
Homogeneous catalysts are typically unsupported and operate in the same phase as the reactants of the reaction they catalyse. Therefore their preparation does not require any step(s) for immobilising the catalytically active components onto a solid support, and their addition to, and mixing with, the reactants of the reaction they catalyse is much easier. However, separation of the catalyst from the reactants becomes more difficult, and in some cases not possible. This means that, in general, homogeneous catalysts either require to be replenished more often than heterogeneous catalysts, and/or additional steps and hardware are required in the process to remove the catalyst from the reactants and reaction products, with an obvious impact on the overall economy of the processes that they catalyse.
Regarding the two-step continuous process of making glycols from saccharide-containing feedstock, as described in WO2015028398, the activities and robustness of the at least two catalytic components, each of which is typically a heterogeneous catalyst, can vary with respect to each other, and therefore if the activity of any one of them declines sooner than the activity of the other, the process of glycol production will not go to completion as efficiently as it was at the commencement of the process, forcing the operators to stop the process to recharge one or both of the catalysts. Alternatively, breakdown components of one of the two catalytic components may adversely affect the other's activity. Again in such a case, the operators of the process are forced to stop the process to recharge one or both of the catalysts. A particular problem is caused by the catalyst component with retro-aldol catalytic capabilities, as over time it degrades and components leach from it. In particular, insoluble tungsten and molybdenum compounds and complexes are formed from with the reactants in the reactor vessel over time. This problem is compounded by the deposition of organic degradation products, sintering of metal particles. Such insoluble matter attach to and clogs up the catalyst component with hydrogenation capability, especially if such catalyst component comprises porous solid support and/or is unsupported, but nevertheless has a porous surface topology (such as Raney-nickel). Further, the catalyst component with hydrogenation capability may also be poisoned by sulphur or other causes.
It would, therefore be, advantageous to be able to prepare an unsupported hydrogenation catalyst, which is suitable for the hydrogenation of retro-aldol fragments in the process for the preparation of glycols from saccharide-containing feedstock: (i) with minimal labour, including without the time consuming and tricky step of immobilisation of the catalytically active components on a solid support, (ii) which functions with the advantages of both a homogeneous-type and a heterogeneous-type catalysts, but without their respective disadvantages and (iii) which is unaffected by insoluble chemical species originating from the degradation of the catalyst component with retro-aldol catalytic capabilities, so that the two-step process of the conversion of saccharide-containing feedstock to glycols can be carried out in one reaction vessel, thus simplifying the process.
Further, in cases where the preparation of glycols from saccharide-containing feedstock is carried in a reactor vessel which was preloaded with a hydrogenation catalyst such as Raney-nickel (i.e. a hydrogenation catalyst other than the unsupported hydrogenation catalyst claimed herein) which is susceptible to the insoluble chemical species generated by the degradation of the catalyst component with retro-aldol catalytic capabilities, it would also be an advantage to be able to prolong reactor runtimes by, for example, being able to supplement the hydrogenation activity in the reactor vessel without stopping and opening up the reactor vessel, simply by, for example, the addition of the second hydrogenation catalyst via the liquid feed intake of the reactor vessel.